Surrounded by hundreds of well-wishers who packed the Pearl Stable to celebrate the twins’ 39th birthday, Congressman Castro announced that he and fiance Anna Flores were expecting their first child in December. In an interview with local ABC affiliate, KSAT, the expectant father reported the couple is expecting a girl.

Castro and Flores confirmed what is probably one of the worst-kept secrets in San Antonio. She broke the news in a Facebook post last month.

Joaquin Castro, a freshman congressman and the younger of the twin set, was dubbed the city’s most eligible bachelor by the New York Times in 2010. On her Facebook page, Flores reports the two met in 2008 and became engaged in June. The mayor offered his congratulations in a June 18 post.

Flores, baby bump in full view, joined San Antonio first lady Erica Lira Castro and daughter Carina, mama Rosie Castro and the birthday boys on stage for cake and candles.

The brothers were serenaded a day earlier, on Sunday in Iowa, where they attended the storied Sen. Tom Harkin Steak Fry–a historically king-making Democratic fundraiser, where headliners have included President Barack Obama and this year, Vice President Joe Biden, who joined the mayor and Harkin on stage.